Piston vibrators have been used to vibrate metal plate and wire electrodes, through rapping rods, in electrostatic precipitators to remove from the electrodes accumulations of precipitated dust. Accumulated dust is undesirable since it loads up the electrodes, short-circuiting them, and reducing the efficiency of the apparatus in the performance of its precipitating function. Various arrangements of this general type are shown in U.S. Pat. Nos. 1,758,404, 2,525,325, 2,552,771, 2,699,224, 2,777,535, 2,854,089, 2,985,802, 3,030,753, 3,570,628, and 3,605,915. Most prior art vibrator arrangements of this type have many drawbacks since they do not effectively transmit the shock wave to the rapper rod because the entire vibrator case must be accelerated. Furthermore, slack often exists between the vibrator housing or strike surface and the rapping rod which dramatically detracts from an effective transmission of force. None of the prior art arrangements have provided a mechanism whereby the rapping rod can be adjustably pre-loaded to tune each individual rapper rod and its corresponding plates or wires.